Insulator for coaxial conductors



Dec. 26, 1933. L. T. WlLSON INSULATOR FOR COAXIAL CONDUCTORS Filed April15, 1951 INVENTOR Z. lFl isol o BY ATTORNEY Patented Dec. 26,1933

UNITED STATES 1,940,780 INSULATOR FOR COAXIAL CONDUCTORS Leon T. Wilson,Chatham, N. J., assignor to American Telephone and Telegraph Company, acorporation of New York Application April 15, 1931. Serial No. 530,388

3 Claims.

This invention relates to coaxial conductor systems and moreparticularly to insulating spacers to be used in separating the coaxialconductors.

Various forms of spacers, including a simple cylinder or disk andmodified desi ns having parts of the dielectric cut out, have beenproposed from time to time for separating the conductors of a concentricconductor system. With these constructions, however, the disk being ofthe same thickness throughout, the flux density increases in passingfrom the outer conductor to the inner conductor due to the fact that thecylindrical area of the dielectric must obviously decrease withdecreasing diameter. It is known that the absorption losses in thedielectric are proportional to the square of the flux density. Thismakes it important to obtain uniform flux density as the the insulatingspacer in order to approximate the condition of uniform flux densitythroughout the material of the spacer.

The invention will now be more fully understood from the followingdescription, when read in connection with the accompanying drawing,Figure 1 of which shows a section of a concentric conductor systememploying one type of insulating spacer in accordance with theinvention; Figs. 2 and 3 show cross-sections taken through two forms ofinsulating spacers which may be used in the conductor system of Fig. 1;Fig. 4 shows a section of concentric conductor system employing adifferent form of insulating spacer in accordance with the presentinvention; Fig. 5 and Fig.6 showing, respectively, side and edge viewsof the insulating spacer employed in Fig. 4.

In a typical concentric conductor system, an outer cylinder 0 ofconductive material (see Fig. 1) surrounds an inner cylinder I ofconductive material, the two cylinders being concentrically arranged andone connected as a return for the other. In such a system theattenuation will be a minimum if the space between the conductors be ofsome gaseous dielectric such as air, which has a dielectric constant ofunity. For mechanical reasons, however, it is necessary to providespacers of solid dielectric materials between the conductors atintervals. Such a spacer is at S in Fig. 1.

The most obvious and simple form of spacer is a right circular cylinderto fit the inside of the outer conductor and having a hole through itscenter to take the inner conductor. Let us now consider the requirementsfor low leakage dielectric constant. A

shown' conductance in'such a spacer of some given material.

First, let us assume that the leakage conductan'ce' is due entirely todielectric absorption in the solid dielectric and that this absorptionis proportional to the square of the flux density. This latterassumption is based on general experimental results with a large numberof solid dielectrics.

The dielectric absorption then is similar to the PR loss in a conductingmedium. In the case of the FR loss, it is well known that the resistanceis a minimum for a given current when that current is uniformlydistributed throughout the medium. Similarly, it is reasonable tosuppose that the dielectric absorption in an insulating material will bea minimum when the flux density is uniform throughout the dielectric.

Now, if a spacer is to contain a certain amount of dielectric,determined, say, by mechanical con- 'siderations, the losses in thatvolume of dielectric should be a minimum if the shape of the dielectricis such as to give as nearly uniform flux density as possible.

In the particular case of a concentric or 00- axial conductor system inwhich a homogeneous dielectric of uniform thickness is interposedbetween the inner and outer conductor, the flux leaves a relativelylarge area of the outside conductor and enters a relatively small areaof the inner conductor. in going from the outer to the inner conductor.To obtain more uniform flux density, it is therefore proposed, inaccordance with one form of the present invention, to increase thecylindrical crosssectional path of the flux as it approaches the innerconductor.

For simplicity, consider a spacer of the shape shown in cross-section inFig. 2, this spacer. extending between an outer conductor 0 ,(seeFig. 1) and an inner conductor I, the concentric axis of the twoconductors being as shown at AA, the spacer being surrounded by a mediumof zero simple calculation shows where these symbols have thesignificance shown in Fig. 2, the cylindrical cross-sectional area atany radial distance from the axis AA will be the Thus, the flux densityincreases conductor.

same as that for any other radial distance. Hence, the flux density willbe uniform at all points in passing from the outer to the innerconductor.

With this arrangement, it will be obvious that as the solid dielectricis of uniform dielectric, constant throughout, and the cylindricalcrosssectional area at any radial distance from the axis is the same,the product of the dielectric constant by the volume of a thincylindrical layer of dielectric at any radial distance from the axiswill be constant.

In actual practice, the medium surrounding th spacer will have a finitedielectric'constant (about unity) and some of the flux leaving the outerconductor through this gaseous medium may enter the solid dielectric andcontinue to the inner It is therefore desirable to increase the axialthickness more rapidly as the inner conductor is approached than wouldbe indicated by the above inverse linear relation. The general shape ofthe spacer would be somewhat as shown in Fig. 3 where r [b i The exactshape will depend on the relative dielectric constants of the spacermaterial and the surrounding medium, the smaller the ratio the greaterthe curvature. It may be quite impos- I sible to obtain absolutelyuniform flux density in the solid dielectric but the general shape shownin Fig. 3 should give more uniform flux distribution than could beobtained by a homogeneous spacer having the same axial thicknessthroughout.

If, for mechanical reasons, such as ease of manufacture, it is desiredto keep the axial thickness of the spacer'uniform, then the insulationmay be graded somewhat as is general in high voltage cables. In thisarrangement the material of highest dielectric constant is employed nextto the inner conductor, and then the next highest, and so on, with thematerial of minimum dielectric constant adjacent the outside conductor.Figs. 4, 5 and 6 show one form of insulator which approximates thedesired graded condition, the insulator being shown applied to the concentric conductors in Fig. 4.

In this case the spacing insulator S assumes the form of alternate ringsof solid dielectric and air dielectric, as shown in Fig. 5. The solidrings are indicated at d1, d2 and ds and the successive rings of gaseousdielectric at m, az and (13. For

. mechanical reasons it is-obviou'sly necessary to ring of airdielectric is made of greater thickness with respect to thecorresponding solid ring as we pass from the inner conductor to theouter conductor.

With this arrangement, the cylindrical crosssectional area decreases aswe approach the axis but the effective dielectric constant increases (ifwe consider the average dielectric constant of two adjacent layers ofdifferent dielectric material). Hence, the product of the volume of acylindrical layer of limited thickness, but including two dielectrics,by the average or eifective dielectric constant of that volume ofmaterial, may be made approximately the same at any radial distance fromthe axis.

Other forms of spacer involving the principles hereinbefore discussedwill readily suggest themselves. In general it appears desirable thatthe surfaces of dielectric in contact with the conductors be smooth andclose-fitting in order to obtain a more uniform flux distribution.Intimacy of contact between the inner conductor and the dielectric isparticularly desirable because of the relatively small area of contactat the inner conducton.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is: a

.1. In a concentric conductor system, a disk-like spacer between theinner and outer conductors, said spacer being so shaped and beingcomposed of such dielectric materials that the product of the volume ofa limited cylindrical ring of the spacer multiplied by the averagedielectric constant of the material of the ring will be substantiallythe same at any radial distance from the concentric axis of the system.

2. In a concentric conductor system, a disklike spacer between the innerand outer conductors, the linear thickness of said spacer being sorelated to the dielectric constant and to the circumferential dimensionat each radial distance from the concentric axis that the flux densitywill be substantially the same at any radial distance from'theconcentric axis of the system. r

3. In a concentric conductor system, a disk-like spacer between theinner and outer conductors, said spacer having its linear thicknessalong the concentric axis increased as the axis is approached inaccordance with the dielectric constant-and the circumferentialdimension at each radial distance from the concentric axis, so that theflux density will be substantially the same at any radial distance fromthe axis.

LEON T. WILSON.

